Projector optical device

ABSTRACT

An object of the present invention is to provide a projector optical device in which both of an image forming section and a screen are held stationary in their fixed locations and an object image distance is constant, wherein an effect from a variation of optical characteristics caused by the heat, such as a variation of focal distance of a lens in a projection lens system located in the vicinity of a light source, can be eliminated and thereby a clear image in the initial focused condition can be maintained on the screen. Provided is A projector optical device including a retro-focus projection lens system characterized in that at least one or more lenses of a group of convex lenses in said retro-focus projection lens system located in an light source side is supported by a compensation member capable of expanding or contracting in the direction of an optical axis in response to a temperature change, wherein an object image distance being variated by a variation of a focal distance of said group of convex lenses caused by a temperature change is compensated for by difference of the expansion or the contraction of said compensation members resultant from the temperature change.

FIELD OF THE INVENTION

The present invention relates to a projector optical device having afear that a lens system could be subject to a temperature change,including, for example, a rear projection-type imaging unit, such as arear projection television and a rear projector, and a front projectorto be used in a home theater environment, all of which have a lenssystem and an exothermic light source. In particular, the presentinvention relates to a projector optical device which allows apredetermined clear image to be formed continuously even if the lenssystem is subject to a temperature change and thereby causes a variationof its optical performance, by compensating for said variation.

DESCRIPTION OF THE PRIOR ART

In both of the rear projector and the front projector, considering fromthe optical viewpoint, once they have been provided with a focusadjustment, an “object image distance” defined by an optical distancebetween an image forming section, such as a liquid crystal, and a screendisposed in an image-forming site is kept to be constant. On one hand,such projector includes as a light source a Xenon lamp or the likegenerally exhibiting high heating value. A heat radiation from thislight source have much effect of high temperature especially on a lenslocated in the vicinity of the light source of the projection opticalsystem.

On the other hand, recently, in order to project the optically excellentvideo images that are free from any types of aberration, an anomalousdispersion glass lens and/or a plastic lens have been employed in theprojection optical system. The variation of the optical performance ofthe anomalous dispersion glass lens caused by the temperature change isshown in Table 2 by taking the reference values designed at a referencetemperature of 20° C., which will be described later, and it has beenmade obvious that the optical performance is deteriorated by thetemperature change.

Conventionally, a temperature compensated optical device has beensuggested, in which a focal distance variation of the lens resultantfrom the temperature rise can be compensated for (see for example, thePatent Document No. 1). That is, in this optical device (i.e., acoliimator) composed of a lens barrel whose length would be changed inassociation with the temperature change and a lens held in this lensbarrel, the lens is made of such a material that allows the focaldistance to be changed in response to the temperature change but thechange in the focal distance is offset by the change in the barrellength of the lens barrel, so that the focusing position of the lens canbe held constantly in a certain position. With this configuration in theoptical device composed of the lens barrel and the lens, such atemperature compensated optical device can be obtained that can preventa fluctuation of the focusing position as of the optical device even ifthe length of the lens barrel has been changed due to the temperaturechange.

Further, such a camera has been conventionally suggested, in which thevariation of the focusing position of a camera lens due to thetemperature change can be mechanically offset and compensated for byusing a material having a degree of freedom in selection (see, forexample, Patent Document No. 2). That is, provided is a cameracomprising: a lens frame for holding a camera lens system including aplastic lens; a lens barrel for fixing the lens frame onto a lens framefixing portion; an elastic member disposed substantially in parallelwith the lens barrel frame and fixed to an immobilized fixing member ina site located in an image-forming plane side defined by the cameralens; and a connecting member disposed substantially in parallel withthe elastic member and connected to the lens barrel in a site located inthe image-forming plane side with respect to the lens frame fixingportion and to the telescopic member in a site located in the objectside with respect to the fixing portion to the fixing member,respectively, so as to support the lens barrel, wherein both of the lensbarrel and the elastic member are made of material having a largecoefficient of linear expansion, while the lens frame and the connectingmember are made of material having a small coefficient of linearexpansion.

[Patent Document No. 1] Japanese Patent Laid-open Publication No.Hei6-130267

[Patent Document No. 2] Japanese Patent Laid-open Publication No.2003-185904

The optical devices as disclosed in the Patent Documents Nos. 1 and 2are designed to compensate for the variation of the focal distance ofthe entire projection lens system. Accordingly, it is true that thetechnologies disclosed in the above Patent Documents are effective tocompensate for the variation of the focusing position, but since thefocal distance of the projection lens system has varied and thereby thelocation of image-forming has been also shifted, therefore the clearimage achievable in the initial focused condition could not be obtainedunless the screen position is correctly shifted.

On one hand, in the projector optical device described above, after thefocusing adjustment having been performed at an initial stage, typicallythe image forming section and the screen are held stationary in theirfixed locations, and accordingly the clear image achievable in theinitial focused condition could not be maintained on the screen. Thatis, for example, in case where the focal distance of the projection lenssystem has become longer due to the temperature rise, even if thedistance between the projection optical system and the image-formingsection is adjusted to be longer such that the focusing position may beplaced in a proper location, in contrast with an expectation that thespace between the projection lens system and the screen should becomelonger in response to the increase in the focal distance of theprojection lens system, actually said space between the projection lenssystem and the screen is made shorter by a distance equivalent to thedisplacement of the projection lens toward the screen side, andaccordingly the clear image achievable in the initial focused conditioncould not be maintained on the screen.

SUMMARY OF THE INVENTION

The present invention has been made in the light of the above-pointedproblems in association with the projector optical devices according tothe prior art, and an object thereof is to provide a projector opticaldevice in which both of an image forming section and a screen are heldstationary in the object image distance is constant, wherein an effectfrom a variation of optical characteristics caused by the heat, such asa variation of the focal distance of a lens in a projection lens systemlocated in the vicinity of a light source, can be eliminated and therebya clear image in the initial focused condition can be maintained on thescreen.

The present invention provides a projector optical device including aretro-focus projection lens system characterized in that at least one ormore lenses of a group of convex lenses in said retro-focus projectionlens system, located in an light source side, is supported by acompensation member capable of expanding or contracting in the directionof an optical axis in response to a temperature change, wherein anobject image distance being variated by a variation of a focal distanceof said group of convex lenses caused by a temperature change iscompensated for by difference of the expansion or the contraction ofsaid compensation members resultant from the temperature change.

Preferred embodiments of the present invention will be described below.

An embodiment of the present invention is characterized in that saidgroup of convex lenses includes an anomalous dispersion glass lens.

An alternative embodiment of the present invention is characterized inthat said group of convex lenses includes a plastic lens.

An another alternative embodiment of the present invention ischaracterized in that said projector optical device is a frontprojector.

An another alternative embodiment of the present invention ischaracterized in that said projector optical device is a rear projector.An another alternative embodiment of the present invention ischaracterized in that said compensation members comprise

-   -   a first elastic lens barrel fixed to a fixing lens barrel which        is fixed to the projector optical device and extending toward        the light source side, and    -   a second elastic lens barrel fixed to said elastic first lens        barrel at the light source side concerning a fixed position of        said elastic first lens barrel to said fixing lens barrel,        extending toward a screen and supporting at least a part of a        group of convex lenses in said retro-focus projection lens        system at its front portion, wherein a coefficient of linear        expansion of said second elastic lens barrel is larger than one        of said first elastic lens barrel.

An another alternative embodiment of the present invention ischaracterized in that said first elastic lens barrel has substantiallythe same length as said second elastic lens barrel in the direction ofoptical axis.

An another alternative embodiment of the present invention ischaracterized in that a micro-display is supported at a rear portion ofa fixing lens barrel extending toward the light source.

FUNCTIONAL ADVANTAGES OF THE INVENTION

According to a projector optical device of the present invention, suchan effect can be obtained that in the projector optical device in whichboth of the image forming section and the screen are held stationary intheir fixed locations and the object image distance is constant, theeffect from the variation of the optical characteristics caused by theheat, such as the variation of the focal distance of the lens in theprojection lens system located in the vicinity of the light source, canbe eliminated and thereby the clear image in the initial focusedcondition can be maintained on the screen.

Namely, according to a projector optical device of the presentinvention, it is assumed that temperature in an area around theretro-focus projection lens becomes higher under the condition which theclear image in the focused condition is maintained on the screen. Backfocal distance BF of the retro-focus projecting lens is lengthen andthen an image projected from the projection lens is made over thescreen, namely at the area of the screen opposite to the area thereoffacing toward the projection optical device. Meanwhile the at least oneor more lenses of a group of convex lenses in the retro-focus projectionlens system located in the light source side is moved toward the screenin accordance with the difference of the expansion or the contraction ofsaid compensation members resultant from the temperature change. As aresult the object image distance is shortened and the clear image in theinitial focused condition is realized on the screen again.

Further, according to a projector optical device of the presentinvention, even if the at least one or more lenses of a group of convexlenses in the retro-focus projection lens system, located in the lightsource side, is located at a position where a lens barrel supports it,in the condition that the first lens barrel has substantially the samelength as that of the second lens barrel in the direction of the opticalaxis, it is possible to move the at least one or more lenses of a groupof convex lenses in the retro-focus projection lens system toward thescreen by the predetermined distance on the basis of difference ofelongations between the first elastic lens barrel and the second elasticlens barrels. As a result the harmful effects of elongation of the backfocal distance BF caused by heating is removed, namely the object imagedistance is shortened and the clear image in the initial focusedcondition is realized on the screen again.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an optical system and relating componentsin a projector optical device according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A projector optical device according to an embodiment of the presentinvention will now be described with reference to the attached drawings.The projector optical device 10 comprises a projection lens system 20 ofretro-focus type specified by a focal distance of 8.8 mm, a prism system30 and a micro-display 40 serving as an image-forming section, as shownin FIG. 1. A light source (not shown in FIG. 1) is located behind themicro-display 40. Magnification of projection is 79 to 106. Theprojection lens system 20 comprises a group of front lenses 22consisting of lenses G1 to G3 and specified by the focal distance of−27.6 mm and a group of rear lenses 24 consisting of lenses G4 to G11and specified by the focal distance of +43.26 mm. The lens G3 is aplastic lens. The lenses G8, G10 and G11 are anomalous dispersion glasslenses. A screen (not shown in FIG. 1) is located in front of the groupof front lens 22.

The above-described optical components are supported by a first lensbarrel 50, a second lens barrel 52, a third lens barrel 54 and a fourthlens barrel 56. The first lens barrel 50 supported by a projectoroptical device main body (not shown) supports the second lens barrel 52as well as a first elastic lens barrel 100. The first lens barrel 50also supports the micro-display 40 at its rear portion. The second lensbarrel 52 supports the third lens barrel 54 as well as the lenses G4 andG5 and an aperture A. The third lens barrel 54 supports the fourth lensbarrel 56. The fourth lens barrel 56 supports the lenses G1 to G3.

The first elastic lens barrel 100 supported in its front end portion bya screen side end portion or an front end portion of the first lensbarrel 50 supports in its rear end portion a rear end portion of asecond elastic lens barrel 110. The second elastic lens barrel 110, inturn, supports in its front end portion the lenses G6 to G11 via a lensframe 112. The focal distance formed by the lenses G6 to G11 is −80.65mm. The first elastic lens barrel 100 and the second elastic lens barrel110 operate as compensation members for compensating for a displacementalong an optical axis and a change in the focal distance of said groupof convex lenses caused by the temperature change.

The first lens barrel 50, the second lens barrel 52, the third lensbarrel 54 and the fourth lens barrel 56 are made of brass, aluminum andthe like. The first elastic lens barrel 100 is made of PPS having acoefficient of linear expansion of 18×10⁻⁶. The second elastic lensbarrel 110 is made of PC having a coefficient of linear expansion of7×10⁻⁶.

In an alternative embodiment, the first elastic lens barrel 100 may bemade of PC-G30% having a coefficient of linear expansion of 30×10⁻⁶. Thesecond elastic lens barrel 110 may be made of ABS having a coefficientof linear expansion of 12×10⁻⁵.

Calculated values of the compensation for the temperature change in theabove projector optical device 10 are as indicated in Table 1. In Table1, an amount of out-of-focus indicates a difference in position of animage-forming position relative to a screen position. The coefficient oflinear expansion of the brass used in the first lens barrel 50 is18×10⁻⁶ and the coefficient of linear expansion for the aluminum is23.6×10⁻⁶. “First lens barrel expansion ignored” in Table 1 indicatesthat the First lens barrel 50 is made from ceramics and the like. TABLE1 Amount of out-of-focus First lens barrel made First lens barrel madeTemp. in the front side Temp. in the rear side (First lens barrel ofbrass of aluminus of the aperture of the aperture expansion ignored)Amount of out-of-focus Amount of out-of-focus 20° C. 20° C. 0.0 μm 0.0μm 0.0 μm  0° C.  0° C. −78.3 μm −34.4 μm −20.7 μm 40° C. 40° C. 80.5 μm36.6 μm 22.9 μm 50° C. 50° C. 120.8 μm 54.9 μm 34.4 μm 60° C. 60° C.163.0 μm 75.2 μm 47.8 μm 40° C. 50° C. 126.7 μm 60.8 μm 40.3 μm 40° C.60° C. 174.7 μm 86.9 μm 59.5 μm 50° C. 60° C. 168.8 μm 81.0 μm 53.6 μm

Further, in a prototype of the above-described projector optical device10 shown in FIG. 1, a distance FD between an article mounting surface(flange surface) and a rear end surface of lenses and a metal-backedvalue MB and a flange-backed value FB relative to a reference taken at20° C. were the distance FD of 47.452 mm (deviation by −0.022 mm) andthe MB of 123.602 mm (deviation by −0.030 mm), respectively, at 0° C.The distance FD of 47.542 mm (deviation. by +0.068 mm) and the FB of123.691 mm (deviation by +0.059 mm) were observed at 50° C.

In the above mentioned embodiment the first lens barrel 50 is made ofbrass or aluminum which have a small coefficient of linear expansion. Itis known from Tables 1 and 2 relating to the above mentioned structurethat the micro-display 40 is moved rearwardly to a certain extent. Theobject image distance is slightly shortened by such a first lens barrel50 so that the amount of out-of-focus is slightly decreased. As a resultthe harmful effects of elongation of the back focal distance BF causedby heating has been removed to a certain extent.

It can be easily estimated by one in the person skilled in the art that,if the first lens barrel 50 is made of PC having a coefficient of linearexpansion of 7×10⁻⁵ or ABS having a coefficient of linear expansion of12×10⁻⁵, the micro-display 40 is sufficiently moved rearwardly. Theobject image distance is appropriately shortened by such a first lensbarrel 50 so that the amount of out-of-focus is thoroughly decreased. Asa result the harmful effects of elongation of the back focal distance BFcaused by heating has been effectively removed.

COMPARATIVE EXAMPLE

In the comparative example the first elastic lens barrel and the secondelastic lens barrel are not used and the lens barrel 112 supporting thelenses G6˜G11 is directly mounted on the second lens barrel 52. “Firstlens barrel expansion ignored” in Table 2 indicates that the First lensbarrel is made from ceramics and the like. TABLE 2 Amount of out-of-First lens barrel made First lens barrel made focus of brass ofaluminums Temp. in the front Temp. in the rear side (First lens barrelAmount of out-of- Amount of out-of- said of the aperture of the apertureexpansion ignored) focus focus 20° C. 20° C. 0.0 μm 0.0 μm 0.0 μm  0° C. 0° C. −120.0 μm −76.1 μm −62.4 μm 40° C. 40° C. 122.2 μm 78.3 μm 64.6μm 50° C. 50° C. 184.0 μm 118.1 μm 97.6 μm 60° C. 60° C. 247.4 μm 159.6μm 132.2 μm 40° C. 50° C. 189.9 μm 124.0 μm 103.5 μm 40° C. 60° C. 259.2μm 171.4 μm 144.0 μm 50° C. 60° C. 253.3 μm 165.5 μm 138.1 μm

INDUSTRIAL APPLICABILITY

Although the present invention has been described with reference to theembodiment representing the projection lens system of retro-focus type,the technological concept thereof can be embodied effectively even inthe lens system of other types, including, for example, the lens systemof Gauss-type or Petzval-type.

1. A projector optical device including a retro-focus projection lenssystem characterized in that at least one or more lenses of a group ofconvex lenses in said retro-focus projection lens system located in anlight source side is supported by a compensation member capable ofexpanding or contracting in the direction of an optical axis in responseto a temperature change, wherein an object image distance being variatedby a variation of a focal distance of said group of convex lenses causedby a temperature change is compensated for by difference of theexpansion or the contraction of said compensation members resultant fromthe temperature change.
 2. A projector optical device in accordance withclaim 1, in which said group of convex lenses includes an anomalousdispersion glass lens.
 3. A projector optical device in accordance withclaim 1, in which said group of convex lenses includes a plastic lens.4. A projector optical device in accordance with claim 1, in which saidprojector optical device is a front projector.
 5. A projector opticaldevice in accordance with claim 1, in which said projector opticaldevice is a rear projector.
 6. A projector optical device in accordancewith claim 1, in which said compensation members comprise a firstelastic lens barrel fixed to a fixing lens barrel which is fixed to theprojector optical device and extending toward the light source side, anda second elastic lens barrel fixed to said elastic first lens barrel atthe light source side concerning a fixed position of said elastic firstlens barrel to said fixing lens barrel, extending toward a screen andsupporting at least a part of a group of convex lenses in saidretro-focus projection lens system at its front portion, wherein acoefficient of linear expansion of said second elastic lens barrel islarger than one of said first elastic lens barrel.
 7. A projectoroptical device in accordance with claim 6, in which said first elasticlens barrel has substantially the same length as said second elasticlens barrel in the direction of optical axis.
 8. A projector opticaldevice in accordance with claim 1, in which a micro-display is supportedat a rear portion of a fixing lens barrel extending toward the lightsource.